Led by Motorola's Advanced Technology and Projects group, Project Ara is developing a free, open hardware platform for creating highly modular smartphones. We want to do for hardware what the Android platform has done for software: create a vibrant third-party developer ecosystem, lower the barriers to entry, increase the pace of innovation, and substantially compress development timelines.

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The design for Project Ara consists of what we call an endoskeleton (endo) and modules. The endo is the structural frame that holds all the modules in place. A module can be anything, from a new application processor to a new display or keyboard, an extra battery, a pulse oximeter - or something not yet thought of!

This could be the biggest innovation in smartphones since, well, the first smartphones appeared on the market. I am incredibly excited about this.

This is dictated by characteristics like power consumption, number of memory channels / frequency.
Anyway you need 200+pin connector for memory interface on the back of the phone, instead of 4 shiny pins in Phonebloks concept.